The dissociative recombination of hydrocarbon ions. III. Methyl-substituted benzene ring compounds

The recombination of electrons with cyclic ions produced via ion-molecule r eactions between atomic precursor ions and methyl-substituted benzene ring compounds (toluene, ortho-, and para-xylene and mesitylene) has been studie d at 300 K using a flowing afterglow Langmuir probe-mass spectrometer appar atus. Differing amounts of energy can be deposited into the daughter ions d epending upon which atomic precursor is used. It has been found that same-m ass daughter ions formed from different precursors displayed different reco mbination rate coefficients indicating that different isomeric forms were r eacting. In particular, the benzene ring of the toluene cation expands to a seven-membered ring following isomerization to the cycloheptatriene form. H atom abstraction allows two different isomeric daughter ions to be formed that do not interconvert and that display different recombination rates. A similar behavior was observed for the xylenes and for mesitylene. All reco mbination rates lie in the range from 10(-7) to 10(-6) cm(3) s(-1) and disp lay no apparent relation with size nor with the aromaticity of the ions. (C ) 2000 American Institute of Physics. [S0021-9606(00)01432-X].